Generally, closed loop (CL) MIMO communications may be defined as a precoding technique wherein an associated precoder is determined by a channel realization and/or interference precoding/channel realization. In order to facilitate the precoding in CL MIMO, channel state information (CSI) may be required. If channel reciprocity is not available, such as in frequency division duplex (FDD) communications systems, then the CSI may need to be fedback from a receiver of the communications (such as a mobile station, also commonly referred to as a user, subscriber, terminal, mobile, User Equipment, and so on) to a transmitter of the communications (such as a base station, also commonly referred to as a controller, NodeB, enhanced NodeB, and so forth). In time division duplex (TDD) communications systems, channel reciprocity may be assumed and CSI feedback may not be necessary; and the CSI may be determined from an alternate channel using channel reciprocity.
With the CSI available at the transmitter, either through channel reciprocity or explicit CSI feedback, the precoder is expected to fit the channel realization and/or the interference environments to achieve better beamforming/diversity gain. However, due to imperfections of CSI feedback, e.g., due to feedback delay, transmission error, quantization error, channel variation, and so on, the precoder and therefore CL MIMO might not be able to practically fulfill the performance potential and sometimes the communications system performance may be severely degraded.
Conventional OL MIMO, on the other hand, may be defined as a precoding technique wherein the precoder is predetermined at the transmitter and is independent of channel realization. Unlike CL MIMO, conventional OL MIMO does not require CSI information (either through channel reciprocity or explicit feedback) and therefore is more robust to channel variations in time, frequency, and/or space domains. However, conventional OL MIMO may not exploit any CSI at the transmitter and therefore, the corresponding performance is limited.